1. The Field of the Invention
The present disclosure relates in general to real time 3D subsurface imaging of electrical conductivity using devices with electromagnetic (EM) sources and/or EM sensors mounted on at least one moving platform to generate EM data.
2. The Related Technology
Electromagnetic (EM) geophysical surveys are widely used in mineral, hydrocarbon, geothermal and groundwater exploration, in-situ mining, hydrocarbon, geothermal and groundwater resource monitoring, unexploded ordinance (UXO), improvised explosive device (IED), tunnel, and underground facility (UGF) detection, geosteering, bathymetry mapping, ice thickness mapping, and environmental monitoring. The state of the art in EM geophysical surveying has been described by Zhdanov, 2009.
To provide economical reconnaissance of subsurface geological structures and man-made objects, EM sources and/or EM sensors are often deployed from moving platforms such as vessels, wireline devices, bottom hole assemblies (BHAs), vehicles, airplanes, helicopters, airships, and unattended aerial systems (UAS).
For example, airborne EM surveys from fixed wing aircraft typically acquire 500 line km of data each day, and airborne EM surveys from helicopters typically acquire 200 line km of data each day. Airborne EM surveys typically contain multiple survey lines that aggregate as hundreds to thousands of line kilometers of EM data measured every few meters and cover an area hundreds to thousands of square kilometers in size.
Over the last twenty years, airborne EM systems have evolved with ever higher moments, and hardware and processing technologies have improved data quality significantly. Nevertheless, very few discoveries of economic mineral deposits have been directly attributed to airborne EM. Modern airborne EM systems can provide real time acquisition and processing of global positioning system (GPS) located and time synchronized EM data. However, the nonlinear physics of EM implies that these raw EM data cannot be transformed for direct characterization of the subsurface conductivity in real time. The state of the art airborne EM interpretation is not real time and is based on various 1D methods such as conductivity depth images (CDIs), conductivity depth transforms (CDTs), layered earth inversions, laterally constrained layered earth inversions, and spatially constrained layered earth inversions which cannot reliably or accurately capture the geological complexity of the 3D subsurface conductivity. Therefore, a need exists in airborne EM interpretation resulting in 3D volume imaging of EM data. Given the time sensitivity of today's economic environment for mineral exploration, there exists a need in airborne EM surveying with real time 3D volume imaging of EM data to improve mineral exploration success.
As another example, formation evaluation based on borehole electrical conductivity measurements is one of the geophysical methods applied to hydrocarbon exploration and production. From the original wireline induction logging devices, technologies have evolved such that EM systems are now deployed in bottom hole assemblies (BHAs) for logging-while-drilling (LWD) and measurement-while-drilling (MWD). State of the art interpretations of LWD and MWD EM data are based on various 1D methods which cannot reliably or accurately capture the geological complexity of the 3D subsurface conductivity. Moreover, there is considerable interest in the development of “look ahead” or imaging-while-drilling technologies that will optimize well placement for improved oilfield production rates. Given very limited data transfer rates from the BHA to the operator, there is a need to develop real time 3D volume imaging of EM data at the BHA so that 3D volume images of the formations around and ahead of the BHA can be transmitted to the drilling operator in real time.
Subsurface imaging has applications beyond resource exploration and production. EM geophysical methods have been developed for detecting unexploded ordinance (UXO) that contain metal and/or electronic parts. While this UXO detection is relatively mature discipline for weapons test site remediation, improvised explosive devices (IEDs) made primarily from fertilizer and lacking metal or electronic parts represent a persistent direct threat to civilian and military personnel in combat zones. IEDs are far more difficult to detect than standard UXO. There exists an urgent need for real time 3D volume imaging of EM data measured from UASs and vehicles for IED detection and discrimination.
The development and use of high-resolution airborne and satellite surveillance has prompted the widespread proliferation of covert tunnels and underground facilities (UGFs). UGFs are used to produce and harbor both weapons and illegal drugs, and in the case of tunnels, move contraband and people without detection across international borders. There exists an urgent need for real time 3D volume imaging of EM data for tunnel and UGF detection and monitoring.